Focused ultrasound (i.e., acoustic waves having a frequency greater than about 20 kiloHertz) can be used to image or therapeutically treat internal body tissues within a patient. For example, ultrasound waves may be used in applications involving ablation of tumors, thereby eliminating the need for invasive surgery, targeted drug delivery, control of the blood-brain barrier, lysing of clots, and other surgical procedures. During tumor ablation, a piezoceramic transducer is placed externally to the patient, but in close proximity to the tissue to be ablated (i.e., the target). The transducer converts an electronic drive signal into mechanical vibrations, resulting in the emission of acoustic waves. The transducer may be geometrically shaped and positioned along with other such transducers so that the ultrasound energy they emit collectively forms a focused beam at a “focal zone” corresponding to (or within) the target tissue region. Alternatively or additionally, a single transducer may be formed of a plurality of individually driven transducer elements whose phases can each be controlled independently. Such a “phased-array” transducer facilitates steering the focal zone to different locations by adjusting the relative phases among the transducers. As used herein, the term “element” means either an individual transducer in an array or an independently drivable portion of a single transducer. Magnetic resonance imaging (MRI) may be used to visualize the patient and target, and thereby to guide the ultrasound beam.
During a focused ultrasound procedure or an ultrasound imaging, small gas bubbles (or “microbubbles”) may be generated in the liquid fraction of the target tissue, e.g., due to the stress resulting from negative pressure produced by the propagating ultrasonic waves and/or due to rupture of the heated liquid and its accumulation of gas/vapor. Depending upon the amplitude of the applied stress from an acoustic field, the microbubbles may collapse (this mechanism is called “cavitation”) and cause various thermal effects in the target and/or its surrounding tissue. For example, at a low acoustic pressure, stable cavitation of microbubbles may be induced to enhance energy absorption at the ultrasound focal region. Stable cavitation can allow tissue within the focal region to be heated faster and more efficiently than would occur in the absence of microbubbles. At a high acoustic pressure, however, unstable (or inertial) cavitation of the microbubbles may be induced, and this may cause undesired bio-effects such as hemorrhage, cell death, and extensive tissue damage beyond that targeted.
Accordingly, there is a need to detect and monitor microbubble cavitation resulting from therapeutic ultrasound waves so as to adjust a treatment plan to achieve desired therapeutic bio-effects on the target tissue without damaging the non-target tissue.